1. Field
This invention relates to differential signals, and more specifically to matching the rise and fall times of differential signals.
2. Background
Differential signaling methods are becoming increasingly common in high speed digital systems. This is primarily due to the superior timing performance and noise rejection produced using differential signaling methods. However, these advantages do not come without risk. In the effort to produce very high speed differential input circuits, common mode crossing voltage range is often sacrificed. Common mode crossing voltage range is the absolute voltage range over which two differential signals are allowed to cross.
In these cases, having output drivers that produce differential signals that fall and rise at very equal rates helps guarantee that the differential signals cross at a midpoint between a stable high and a stable low state. The problem of differential signals not crossing at the midpoint can become especially detrimental in signaling technologies that appear characteristically unbalanced to the output drivers, e.g., tri-state signaling technologies (e.g., Gunning Transceiver Logic (GTL)). GTL switching (and other tri-state technologies) makes use of pull-up resistors on every signal line, but no pull-down resistors, resulting in an unbalance load to the drivers.
In signaling that has a pull-up but no pull-down, when the driver is pulling the signal line down, this goes against the pull-up resistor which is pulling the signal line up. This, therefore, retards the falling edge (i.e., slowing down) of the signal. When a driver output is driving high, the pull-up helps pull the signal line high thus creating a faster rising edge. When the rising edge of one differential signal does not match the falling edge of the other, an off center crossing voltage results. Input amplifiers are most sensitive at a centered crossing voltage and, therefore, when the crossing voltage is at or near the center of the falling edge of one signal and the rising edge of the other, the system runs cleaner and faster.
Varying the bias on output pre-drivers is a technique that has been used for years to loosely control rise and fall times. However, currently there is no system or method that uses any kind of feedback system to control this bias to make edge rates match. That is, currently there is no circuit that matches rising edge and falling edges so that the signals cross as close to the center as possible.
Therefore, there is a need to provide a system and method to guarantee that rise time matches fall time to within a small area, even in spite of unbalance lines, by providing a control signal that tells whether a particular edge should be accelerated or decelerated.